1. Field of the Invention
The present invention relates to a semiconductor light emitting device and a fabrication method thereof. Particularly, the present invention relates to a semiconductor light emitting device employing a light emitting element such as a light emitting diode (LED), and a fabrication method thereof.
2. Description of the Background Art
An example of a conventional semiconductor light emitting device is shown in FIG. 8.
Referring to FIG. 8, a resin portion 3 is formed by insert-molding in a manner with a first lead frame 1 and a second lead frame 2 secured. An LED chip 4 is mounted on first lead frame 1 with Ag paste 7. A bonding wire 5 is attached to second lead frame 2. LED chip 4 is surrounded by epoxy resin 6 to be protected and sealed.
A fabrication method of the semiconductor light emitting device of FIG. 8 will be described hereinafter.
First and second lead frames 1 and 2 are formed in a predetermined pattern configuration, and embedded in resin portion 3 by insert-molding with the coat of bonding Ag applied. Then, LED chip 4 is mounted on first lead frame 1 with Ag paste 7. Bonding wire 5 is attached to second lead frame 2. Thus, electrical and mechanical connection of the lead frame is established. Sealing is provided by epoxy resin 6. Since the surface of the lead frame will be covered with rust to impede soldering if the surface is left with the Ag coat, an outer coat such as a solder coat is applied on the lead frame region. Then, unnecessary regions of the lead frame are cut away. The lead frame is bent to take the shape of a rigid-angle bent C form, and a terminal for junction with the mounting board is formed.
Heat is generated when light is emitted from the mounted LED chip. The generated amount of heat is proportional to the current flowing through the LED chip. The light emitting efficiency of the LED chip becomes lower as the temperature of the LED chip becomes higher, leading to degradation of light. In other words, brighter light cannot be effectively achieved even if a large current is conducted, and the lifetime of the LED chip will become shorter.
An LED chip exhibiting favorable light emitting efficiency even under high current and with favorable lifetime property can be provided by discharging the heat generated from the LED chip efficiently outside to reduce the temperature of the LED chip.
Such a conventional semiconductor light emitting device aimed to improve heat radiation efficiency is disclosed in, for example, Japanese Patent Laying-Open No. 11-46018 (Conventional Example 1), Japanese Patent Laying-Open No. 2002-222998 (Conventional Example 2), Japanese Patent Laying-Open No. 2000-58924 (Conventional Example 3), Japanese Patent Laying-Open No. 2000-77725 (Conventional Example 4), and Japanese Patent Laying-Open No. 2000-216443 (Conventional Example 5).
In Conventional Examples 1 and 2, heat radiation is improved by increasing the surface area of the lead frame. In Conventional Examples 3, 4 and 5, heat radiation is improved by using metal having thermal conductivity greater than that of resin for the substrate material.
However, increasing the surface area of the lead frame at the LED side by taking a nonlinear configuration for the lead frame or increasing the thickness as in Conventional Examples 1 and 2 is relatively limited by the restricted size of the package. The surface area of the lead frame cannot be increased sufficiently by just altering the configuration. Thus, the heat radiation efficiency could not be improved to a sufficient level.
Since the lead frame is to be bent at a subsequent step, the lead frame cannot be made too thick. Furthermore, a thicker lead frame will require greater force in punching out the lead frame from the sheet member, as compared to a thinner frame. Accordingly, the thickness of the mold must be increased to ensure the strength of the mold, leading to a wider gap of the punching part, i.e., between lead frames. Generally a gap equal to the thickness of the lead frame, or at least ¾ the thickness of the lead frame, is required. If the gap between lead frames is increased, sufficient area for bonding cannot be provided, or the frame area will become smaller.
Another approach related to heat radiation efficiency is to increase the contacting area between the lead frame terminal on the part of the LED element and the mounting board. Although this is advantageous from the standpoint of heat radiation, the heat during soldering will be easily conducted to the LED device to adversely affect the reliability.
In the case where metal having higher thermal conductivity than resin is employed for the substrate as in Conventional Examples 3, 4 and 5, there is a possibility of the heat during soldering adversely affecting the reliability of the LED device, likewise the above-described case.